TECTONIC CONTROLS ON LITHOFACIES AND FOSSIL PRESERVATION IN THE DOVE SPRING FORMATION, SOUTHERN CALIFORNIA

The Dove Spring Formation represents middle to late Miocene sedimentary deposits within the El Paso Basin of the Mojave Region. The formation consists of 1800 m of fluvial and lacustrine sediments deposited between 12.5 Ma and 8.5 Ma. A series of radiometrically dated volcanic ash deposits and magnetrostratigraphic correlations provide age control to a resolution of 200 kyr for over 7,000 vertebrate fossils. The mammalian assemblage exhibits significant changes in faunal composition that coincide with tectonic episodes. Changes in vegetation and water availability may have been influenced by the topographic evolution of the basin and resulting variation in depositional environments. I conducted lithofacies analysis to describe the geographic and temporal distribution of major facies associations and evaluate changes in relation to the basin’s tectonic history. I described the depositional environments of fossil localities with greater than ten specimens and tested the hypothesis that faunal change is related to the distribution of facies through time.

Three macrofacies associations at the meter-scale represent channel deposits, channel margins, and floodplains. During extensional tectonic episodes, large channels were dominant with few proximal floodplain deposits. As shearing became the primary tectonic process, the main channel belt was disrupted and extensive floodplains were supported by smaller channels. Within the broad macrofacies classifications, I also documented landscape heterogeneity at the decimeter-scale. These microfacies associations represent deposits from two size classes of channels and three distinct floodplain expressions. Localities in well-drained floodplain deposits contain the highest concentration of fossil specimens, but channel sequences yield higher species richness, particularly for large mammals. Fining-upward sequences within floodplains represent the majority of highly productive fossil horizons, indicating that life habitats near stream channels had the highest preservation potential for terrestrial vertebrates. Despite a strong correlation between species richness and number of localities, changes in the dominant depositional environment are not a strong predictor of fossil productivity.